Fixed issue with processing of EC_dev_remove event

The pool guid and vdev guid received by zfs_agent_post_event(),
which calls zfs_retire_recv(), are normally non-zero. However,
later in this same method they may be unconditionally reset to
zero by the code which is intended to handle multipath, spare
and l2arc vdevs. This will result in the EC_dev_remove not
being handled.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>\
Co-authored-by: Vipin Kumar Verma <vipin.verma@hpe.com>
Signed-off-by: Srikanth N S <srikanth.nagasubbaraoseetharaman@hpe.com>
Closes #11564